The present invention relates to a lens system for projecting an image of microfilm handled in readers or reader-printers.
The frames on a microfilm are in a vertical or horizontal position relative to the film, while information is recorded in the frame in a vertical or horizontal position relative to the frame. If images on the microfilm are projected with the film in a definite position, it is likely that the image will be projected in an inappropriate orientation. For example, information projected as oriented horizontally in reader mode is almost impossible to read. Further if the orientation in which the frame is projected is not in coincidence with the orientation of copy paper in a printer mode, an incomplete copy will be obtained. These objections can be eliminated by rotating the microfilm itself, but the device for holding the film must then be rotated. This requires a complex arrangement.
U.S. Pat. No. 3,907,418 discloses an arrangement wherein the film image to be projected is rotated on a plane of projection by a prism. The prism employed is a trapezoidal prism, i.e., Dove prism, and is disposed at the light emergent side of a projection lens rotatably about the optical axis of the lens. The rotation of the prism rotates the image through twice the angle of rotation thereof.
Such projection system with the image rotation prism becomes larger in size. Especially, a use of a lens system having an aperture stop at a center position of the lens system or close thereto causes the size of the prism to become much larger because the length crossing the field angle of the lens system becomes larger. In order to solve the drawback mentioned above, Japanese patent publication No. 47-35028 discloses a use of a lens system having an aperture stop provided between the lens and the prism, that is, at the front end of the lens system.
However, it is difficult in general for the front stop type lens systems to compensate for the aberration in the off axial region, and also to compromise with various other aberrations into an adequate performance as a whole because of a limited combination of glass materials for compensating for both longitudinal and lateral chromatic aberration. As apparent from the disclosure of the above mentioned prior art, the lens system thereof has a narrower field angle (about 20.degree.), which results in large-sized projection system.